Chapter 12: Electrochemistry Representing Redox Reactions Galvanic - - PowerPoint PPT Presentation

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Chapter 12: Electrochemistry

Representing Redox Reactions Galvanic Cells Electrolysis The Impact on Materials

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Review from Section K.2 of your Book

Oxidation Number

• 1. The oxidation number (ON) of an element uncombined with another

element is zero: Na(s), C(gr), H2(g), I(s) …

• 2. For monoatomic ions, the charge is the ON: Na+ ON = +1
• 3. The ONs of elements in group 1 equal 1 (ex. Lithium ON = +1) ONs
• f elements in group 2 equal 2 (ex. Magnesium ON = +2)
• 4. The ON of fluorine is always -1 in compounds.
• 5. The ON of the other elements in group 7 usually equals -1
• 6. The oxidation number of oxygen is usually -2 in compounds.

Exceptions are fluorine compounds and peroxides

• 7. Hydrogen's ON is +1 when combine with non metals and -1 when

combine with metals

• 8. The sum of the ONs of all the atoms in a species is equal to its total

charge

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Oxidation Number Examples

H2S ON H = +1 Bonded to a non metal ON S = -2 Total charge must equal O (2(1)-2 = 0) P4O6 ON O = -2 Not bonded to F or forming a peroxide ON P = 3 Total charge must equal O (4(3)-6(2) = 0) NO3

• ON O = -2 Not bonded to F or forming a peroxide

ON N = 5 Total charge must equal -1 (5-3(2) = -1)

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Electroplating

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Corrosion

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O2(g) + 4H+(aq) +4e- 2H2O(l) Eo = 0.82 V 2Fe(s) 2Fe2+(aq) + 4e-

• Eo = 0.44 V

2Fe(s) + O2(g) + 4H+(aq) 2Fe2+(aq) + 2H2O(l) Eo= 1.26 V ½ O2(g) + 2H+(aq) +2e- H2O(l) Eo = 0.82 V 2Fe2+(aq) 2Fe3+(aq) + 2e-

• Eo = -0.77 V

2Fe2+(s) + ½O2(g) + 2H+(aq) 2Fe3+(aq) + H2O(l) Eo = 0.05V 4H2O(l) + 2Fe3+(aq) 6H+(aq) + Fe2O3·H2O(s) Overall Reaction: 2Fe(s) + 3/2O2(g) + H2O(l) Fe2O3·H2O(s)

Corrosion

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KOH Alkaline Electrolyte MnO2 + Carbon Black + NH4Cl Electrolyte Ag2O Carbon Carbon Cathode The batteries are very reliable over time The zinc only reacts with the electrolyte when the battery is being used (longer life) The interior of the container is lined with paper that serves as the porous barrier. The ammonia forms the complex Zn(NH3)4+ with the Zn2+ ions and prevents their buildup. Other Information Zinc Zinc Zinc Anode Medical Implants Longer Lasting Disposable Batteries Disposable Batteries Uses Silver Cell (1.6 V) Alkaline Cell (1.6 V) Dry Cell (1.5 V) Type (emf)

Primary Cells Galvanic cell with the reactants sealed inside at the manufacture. They can not be recharged

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Solid porous aluminum Dilute sulfuric acid Electrolyte Molten S (l) Lead-antimony alloy grid with lead (II) sulfate past on it (during the first charge some of the lead (II) sulfate is oxidized to lead (IV)

• xide)

Cathode Needs temperatures above 320ºC to keep Na and S molten. The battery can generate large amounts

• f current for short periods, such as the

time needed to start an engine. Other Information Molten Na (l) Lead-antimony alloy grid with lead (II) sulfate past on it (during the first charging some of the lead (II) sulfate is reduced to lead) Anode Power Electric Vehicles Car Batteries Uses Sodium Sulfur (2.2 V) Lead-acid (12 V) Type (emf)

Secondary Cells Galvanic cell that must be charged before they can be used. They usually can be recharged

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Lead-Acid Cell Dry Cell

Grids are made of a lead-antimony alloy that are covered in lead(II) Sulfate